Many electronic circuits include a switch, such as a power transistor in switching power converters. The gate of the power transistor is typically coupled to a control signal via a driver. In response to the control signal applied to the input of the driver, the power transistor changes between an “on” state and an “off” state. Although the input control signal of the driver may change instantaneously between a logic “low” and a logic “high”, the output signal of the driver may not change instantaneously because the gate capacitance of a switching element such as a transistor is charged from a low voltage to a voltage beyond the threshold voltage. As known in the art, the voltage across a capacitor cannot change instantaneously. Therefore, the gate voltage may rise from a low voltage to a high voltage in a period. Such a period is determined by the amount of capacitance at the gate and the amplitude of the drive voltage. In order to better understand the gate voltage transition of a switch, gate voltage slew rate is used to estimate the transition speed of a transistor. As known in the art, the gate voltage change in voltage level over time is defined as gate voltage slew rate.
To improve efficiency in power converters, switches are designed to operate as quickly as possible so as to reduce cross conduction losses. However, such fast transitions may cause electromagnetic interference (EMI) noise, which may cause logic defects in digital circuits or noise in audio circuits. The fast transitions may also cause excessive voltage ripple and switching noise. As a result, high slew-rate induced noise may cause malfunction in some noise-sensitive circuits.
In order to reduce high slew-rate induced noise, slew-rate controlled drivers have been developed to reduce a switch's transition speed. In a prior art slew-rate controlled driver, control circuits are utilized to selectively enable or disable “legs/fingers” of the driver. This may cause complex control algorithms and compensation circuits. In another prior art slew-rate controlled driver, a delay compensation circuit is used to control the turning on and off of different “legs and fingers” of the driver. This may cause unnecessary area penalty. In yet another prior art slew-rate controlled driver, a current control circuit is used to control the driver's current. This may cause additional complexity.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.